DOC: minor issue in usage()
DOC: minor issue in usage()

Former-commit-id: 09a540b826aec0a0008894821797ec8764607123 [formerly c0a4ea788cb1618d3b446d9fe96316ef285d6283]
Former-commit-id: 96ca15be13b47035d9fc133e55552d7e5d6c9046
Former-commit-id: c1230297c15eeb05ebe48b747dcd88f1ddabb1e8
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox